Some thirty years ago a young Boston area professional took his wife and small children to a wooded farm in New Hampshire to live the rural life. New England teems with maple trees, some of which bear the sweet sap used for millennia by Indians in making syrup and sugar.
Scene of the Crime
The sap runs in the spring, when a hole must be drilled in the trunk and a tap inserted to transmit the sap from tree to bucket. I do not know how many sugar maples this farm had, but suspect it was in the range from hundreds to thousands. Drilling that many holes by hand is an awful lot of work. But, there is a source of portable power that is ubiquitous in New England: the gasoline chain saw.
A Yankee inventor devised and patented a cast aluminum chuck which fit over the clutch housing on the saw as seen in Figure 1. A drill bit was inserted into the center of the casting, and bingo, a portable gasoline powered drill!
The man in the case was between trees while revving the engine to keep it from dying. The bit snapped near
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Figure 1 |
the chuck and entered his eye. He grabbed the bit and threw it, but the damage was done. The five-inch length of bit was recovered after the snow melted.
The Investigation
I was retained early on by a litigant in the ensuing wrongful death suit and received chain saw and both pieces of the subject bit, along with the exemplar shown in Figure 2. The bit is seen to have bent to an approximately 45 degree angle. Such a deformation indicates a ductile steel.
I was limited to non-destructive testing on the subject bit, which in this case was hardness testing and scanning electron microscopy (SEM) studies. The hardness was in the low 40's on the Rockwell "C" scale, which indicated a steel of relatively high strength along with significant ductility. SEM study showed the desired ductile, dimpled fracture surface. The steel was thus entirely suitable for the use.
The clutch housing was 75-mil-thick pressed steel, and was visibly out of round. Such eccentricity could have been caused by uneven forces from the six set screws that held the chuck
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Figure 2 |
in place or could have been induced during use. When a new bit was inserted, the tip was found to rotate in a 1/4 inch diameter circle. The chain saw engine may turn as fast as 14,000 rpm at no-load, which is an awful lot faster than any electric drill. The resulting centrifugal force at the high rpm would cause the bit to gyrate wildly, inducing bending stresses in the bit.
The Smoking Gun
I concluded that the steel was blameless but that the design was suspect. I am not qualified to delve deeply into mechanical design but know enough to talk to design experts, who in turn know enough metallurgy to talk to me.
Another litigant had a metallurgical and mechanical analysis done by a local testing lab. They confirmed my finding that the steel was sound and suitable.
Their mechanical analysis found that the induced bending stress depended crucially on the rpm and the length of the bit protruding from the chuck.
The bit had only been inserted about 1/2 inch into the chuck at the time of failure, with 5.5 inches protruding. Under these conditions the stresses reached the breaking level well below 14,000 rpm. Had the bit been fully inserted into the chuck, the stresses would have stayed below the breaking level, even at 14,000 rpm.
The taps need only go about two inches into the tree, so that the bit length was excessive. However, the bits seem to be of standard type and were probably what was available at the hardware store. The unfortunate combination of the high-speed engine and the spindly bit was a recipe for disaster. There were, in addition, questions on foreseeable misuse and failure to warn, to use the legal terms.
This case bounced around for about five years and was finally settled out of court. I have no idea as to the size of the settlement or who paid how much.
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